Variable OD coiled tubing strings

ABSTRACT

Coiled tubing or a portion thereof having a first part spaced-apart from a second part, the coiled tubing having a first outer diamter at the first part and a second outer diameter at the second part, the first outer diameter different from the second outer diameter, and outer diameter of the coiled tubing continuously diminishing or increasing from the first part to the second part, in one particular aspect thus varying over its entire length; and methods for using and methods and apparatuses for making such coiled tubing.

RELATED APPLICATION

[0001] This is a division of U.S. application Ser. No. 09/824,451 filedApr. 2, 2001 incorporated fully herein for all purposes.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention is related to coiled tubing for use in wellbores,tubular strings, pipelines, bores, and boreholes.

[0004] 2. Description of Related Art

[0005] Coiled tubing (“CT”) is typically relatively long continuouslengths of pipe, known as strings, which can be run in and out of abore, pipeline, tubular string, borehole, or wellbore and is made, e.g.,of steel or steel alloy, plastic, composites, titanium or othermaterials. The CT is typically stored on a reel.

[0006] CT strings typically have a constant outside diameter (OD) thatdesignates the CT size; e.g., 1.0″, 1.25″, 1.5″, 1.75″, 2.0″, 2⅜″, 2⅝″,2⅞″ and 3.5″ OD's are common. The wall thickness of the string may beconstant, or may vary along the length of the string (known as “tapered”strings. U.S. Pat. No. 4,629,218 describes tapered strings. U.S. Pat.Nos. 4,863,091 and 5,191,911 describes manufacturing processes used forsome conventional steel CT strings.

[0007] Some prior art stepped outer diameter (“OD”) CT strings are madeby connecting sections of CT with different outer diameters. A sectionof pipe with a machined OD varying from one size to the next is used inmaking the transition connection between one size and the next. Forexample, a stepped OD string may be made by connecting 5,000 ft. of 1.5″CT to 5,000 ft. of 1.75″ CT. Another 5,000 ft. of 2.0″ CT is connectedto the 1.75″ forming a 15,000 ft. string made of three sizes of CT. Useof some of these prior art stepped OD strings has been limited dueprimarily to concerns about the bending fatigue life of the string atconnections.

[0008] Certain prior art manufacturing processes for steel CT includethree steps or processes. In the first process, rolls of sheet steelknown as master coils, typically 4 ft. to 6 ft. wide and 1,000 to 3,500ft. long, are slit and into strips which are rolled into slit coils(e.g. like the coils 100 in FIG. 1). The strips in these slit coils arethe length of the original master coil and are the width necessary tomake the particular CT size being manufactured. The thickness of thesestrips may be constant or may vary gradually along the strip length toform a continuous taper as is described in U.S. Pat. No. 4,629,218.

[0009] In the second process, shown in FIG. 1, strips from the slitcoils 100 are welded together at the welder 101, typically using biasedwelds described in U.S. Pat. 4,863,091 and 5,191,911. Once the weld iscompleted and inspected, the strip is spooled onto a large strip reel.Successive strips may have the same thickness as the previous strip, ormay have a different thickness. If the strip thickness differs, thefinal CT will be a tapered string. As multiple strips are weldedtogether, one long continuous strip is made on the strip reel to thedesired length of the CT string, typically between 7,000 and 25,000 ft.long.

[0010] In the third process, shown in FIG. 2, the strip 103 from thelarge strip reel 102 passes through a series of sub-processes whichmanufacture or mill the strip into CT. In the first sub-process 104 thestrip 103 passes through forming rollers that form the strip into a tubeshape 105. These forming rollers are powered so that they pull the stripfrom the large strip reel 103 and move it through the milling process.In the next sub-process 106 the edges of the tube are welded together toform a longitudinal seam weld, typically using an electric resistanceweld (“ERW”) though other weld types may be used. When an ERW process isused an impeder is often placed inside the tube at the point where theweld is being created. As part of this welding process 106 a cutter cutsaway the external weld flash. There may also abe an internal cutter thatcuts away the internal weld flash. In the next sub-process 108 the weldseam of the welded tube 107 is heated to a temperature that normalizesthe grain structure of the material. As part of the seam normalizingprocess 108 the seam is cooled by passing for a period of time throughthe air. In the next sub-process the tube 109 passes through sizingrollers that reduce the tube diameter slightly to its final size. Thesesizing rollers are also powered and help the forming rollers in movingthe tube through the milling process. The sized tube 111 then passesthrough the next sub-process 112 in which it is heated to a stressrelief temperature and then allowed to air cool. After air cooling thetube passes through a water bath for the final cooling 114. Thecompleted tube 115 is then spooled onto a CT reel 116.

SUMMARY OF THE PRESENT INVENTION

[0011] In certain embodiments, the present invention provides coiledtubing strings in which the outer diameter varies continuously or nearlycontinuously over a portion of the string's length. Methods according tothe present invention for making such strings are also disclosed. Thesecontinuously varied OD CT (“VODCT”) strings can be designed forexcellent performance in many situations. In certain aspects they reduceor eliminate bending fatigue problems associated with prior art steppedOD CT strings.

[0012] In certain embodiments VODCT strings according to the presentinvention provide additional strength where it is needed; modify thevelocity and pressure profile in a CT string and/or in the annulusbetween the CT string and the bore; provide the larger diameter whereneeded while meeting weight and size restrictions; have varying wallthickness; and/or provide diameter profile needed when snubbing againsthigh pressures.

[0013] Certain embodiments of this invention are not limited to anyparticular individual feature disclosed here, but include combinationsof them distinguished from the prior art in their structures andfunctions. Features of the invention have been broadly described so thatthe detailed descriptions that follow may be better understood, and inorder that the contributions of this invention to the arts may be betterappreciated. There are, of course, additional aspects of the inventiondescribed below and which may be included in the subject matter of theclaims to this invention. Those skilled in the art who have the benefitof this invention, its teachings, and suggestions will appreciate thatthe conceptions of this disclosure may be used as a creative basis fordesigning other structures, methods and systems for carrying out andpracticing the present invention. The claims of this invention are to beread to include any legally equivalent devices or methods which do notdepart from the spirit and scope of the present invention.

[0014] The present invention recognizes and addresses thepreviously-mentioned problems and long-felt needs and provides asolution to those problems and a satisfactory meeting of those needs inits various possible embodiments and equivalents thereof. To one skilledin this art who has the benefits of this invention's realizations,teachings, disclosures, and suggestions, other purposes and advantageswill be appreciated from the following description of preferredembodiments, given for the purpose of disclosure, when taken inconjunction with the accompanying drawings. The detail in thesedescriptions is not intended to thwart this patent's object to claimthis invention no matter how others may later disguise it by variationsin form or additions of further improvements.

[0015] What follows are some of, but not all, the objects of thisinvention. In addition to the specific objects stated below for at leastcertain preferred embodiments of the invention, other objects andpurposes will be readily apparent to one of skill in this art who hasthe benefit of this invention's teachings and disclosures. It is,therefore, an object of at least certain preferred embodiments of thepresent invention to provide:

[0016] New, useful, unique, efficient, nonobvious coiled tubing stringsor parts thereof with an outer diameter that varies continuously ornearly continuously over all or over a portion of the strings' length;

[0017] Methods for making such CT strings;

[0018] Such strings that can modify velocity and pressure profiles in aCT string; and

[0019] Such strings that provide larger outer diameter in a desiredlocation and/or provide a diameter profile needed when snubbing againsthigh pressure.

[0020] Certain embodiments of this invention are not limited to anyparticular individual feature disclosed here, but include combinationsof them distinguished from the prior art in their structures andfunctions. Features of the invention have been broadly described so thatthe detailed descriptions that follow may be better understood, and inorder that the contributions of this invention to the arts may be betterappreciated. There are, of course, additional aspects of the inventiondescribed below and which may be included in the subject matter of theclaims to this invention. Those skilled in the art who have the benefitof this invention, its teachings, and suggestions will appreciate thatthe conceptions of this disclosure may be used as a creative basis fordesigning other structures, methods and systems for carrying out andpracticing the present invention. The claims of this invention are to beread to include any legally equivalent devices or methods which do notdepart from the spirit and scope of the present invention.

DESCRIPTION OF THE DRAWINGS

[0021] A more particular description of embodiments of the inventionbriefly summarized above may be had by references to the embodimentswhich are shown in the drawings which form a part of this specification.These drawings illustrate certain preferred embodiments and are not tobe used to improperly limit the scope of the invention which may haveother equally effective or legally equivalent embodiments.

[0022]FIG. 1 is a schematic view of a prior art slip assembly method.

[0023]FIG. 2 is a schematic view of a prior art tube miling method.

[0024]FIG. 3 is a top view of a roller mechanism according to thepresent invention.

[0025]FIGS. 4A and 4B are top views of die mechanisms useful in methodsaccording to the present invention.

[0026]FIG. 5 is a schematic view of a drawing method according to thepresent invention.

[0027]FIG. 6A is a side cross-section view of a die mechanism accordingto the present invention. FIG. 6B is a perspective view of part of themechanism of FIG. 6A.

[0028]FIG. 7 is a graphical representation of string according to thepresent invention.

[0029]FIG. 8 is a graphical representation of a string according to thepresent invention.

[0030]FIG. 9 is a schematic view of a tube milling method according tothe present invention.

DESCRIPTION OF EMBODIMENTS PREFERRED AT THE TIME OF FILING FOR THISPATENT Improved Forces Capabilities

[0031] In certain aspects, when CT is run into a well, the point ofmaximum tensile axial force in the CT is typically at the surface, justbelow the stripper or seal which seals around the CT, separating thepressure in the well from the atmosphere. This axial force is caused bythe hanging weight of the CT string in the well. Additional axial forcemay be applied to the CT string when it is being pulled from the welldue to friction between the CT string and the wellbore. This friction isgreater in deviated and curved wellbores. Computer models, called tubingforces models, are often used to calculate the forces in the CT string.28. The axial force (Fa) in the CT causes an axial stress in the CTmaterial. The axial stress (σ_(a)) is defined as the axial force dividedby the cross-sectional area (A) of the CT material.

[0032] There are other stresses in the CT material due to the pressurein the well and in the CT. When the combined stresses become too large,the CT material will yield and eventually fail. Stress limits are setfor various CT materials to prevent CT failures.

[0033] In some cases the CT operational envelope in wells is limitedbecause the axial stress limit is reached. The axial stress can bereduced if the area (A) of the CT material is increased. The increasingwall thickness of tapered strings increases the area (A) and thusexpands the operational envelope of CT. However, the practical amountthat the wall thickness can be increased is limited due to the reductionin the inside diameter (“ID”) of the CT. The increase in the OD withVODCT increases the area (A) of the CT and thus reduces the axialstress. VODCT strings according to the present invention can thus, incertain aspects, increase the operating envelope for CT and, in someaspects, increase it significantly.

[0034] In one example, when running CT into a 30,000 ft. deep emptyvertical well it is assumed that the yield stress (σ_(y)) of the CTmaterial is 100,000 psi and the density (ρ) of the CT material is 0.283lb/in³. For safety purposes the maximum axial stress will be limited to60% of the yield stress. If a simple straight prior art CT string withconstant OD and constant wall thickness is used, the cross-sectionalarea (A) remains constant. In this case the maximum depth the CT can berun to is:${D_{\max} = {\frac{0.6\quad \sigma_{y}}{12\rho} = 17}},{668\quad {{ft}.}}$

[0035] A straight, simple CT string cannot reach the bottom of thisexample well. When using a prior art tapered CT string with a constantOD of 1.5″, five (5) wall thicknesses are used beginning with 0.109″ forthe bottom section of the string, then 0.118″, 0.125″, 0.134″ and 0.156″sections. The 0.109″ section can be 17,668 ft. long before reaching the60% of the yield stress limit according to the equation above. The0.118″ section must bear the load of the 0.109″ section. The 0.118″section can be 1,241 ft. long before the 60% limit is reached. Themaximum depth that this type of tapered string can be run to is 23,135ft., which is still not sufficient to reach the bottom of this well.

[0036] Using a VODCT string according to the present invention thestring is designed so that the axial stress does not exceed 60% of theyield stress. For the first 17,668 ft., the string will be straight 1.5″OD with a 0.109″ wall as discussed above. “ODs” is defined as the OD ofthe straight section, 1.5″ in this example. The OD to thickness ratio(ξ) for this straight section is 13.76. From 17,668 ft. to 30,000 ft.,the OD of the string increases gradually. The wall thickness alsoincreases gradually, so that the OD to thickness ratio (ξ) remainsconstant. For the axial stress to remain constant at 60% of the yieldstress, the OD between 17,668 ft and 30,000 ft. is:${OD} = {\frac{\pi}{4}^{a{({x + x_{0}})}}}$$a = {{\frac{1}{D_{\max}}x_{0}} = {{D_{\max}\left\lbrack {{\log_{n}\left( {\frac{4}{\pi}{OD}_{s}^{2}} \right)} - 1} \right\rbrack} = {927.5\quad {{ft}.}}}}$

[0037] x=the length along the string between 17,668 and 30,000 ft.

[0038] The wall thickness at any point in the string can be calculatedby: $t = {\frac{OD}{\xi} = \frac{OD}{13.76}}$

[0039] In this example, the OD of the CT string at 30,000 ft. is 2.126″and the wall thickness is 0.155″. FIG. 7 shows the profile of thisexemplary VODCT string according to the present invention that willreach 30,000 ft. without exceeding 60% of the yield stress.

[0040] A similar type of VODCT string according to the present inventionis for very long extended reach wells where the friction forces becomeso large that a conventional CT string may not be able to be pushed intothe well (due to insufficient bending stiffness to avoid helicalbuckling) or pulled form the well (due to excessive axial stress) withinthe safe operating limits of the CT material.

Reduced Snubbing Force

[0041] When the end of a CT string is first inserted into the well itmust be pushed or “snubbed” into the well against a “snubbing force”. Incertain embodiments using a CT string according to the present inventionresults in reduced snubbing forces. The wellhead pressure multiplied bythe cross-sectional area of the CT that is in the stripper is an upwardforce (snubbing force), which is trying to push the CT out of the well.As the CT is run deeper into the well, the hanging weight of the stringbecomes sufficient to overcome the snubbing force, and no more snubbingis required.

[0042] The equipment used to run CT in and out of wells is typicallydesigned primarily for applying an upward tensile force on the CT, andis limited in the amount of downward compressive snubbing force it canapply. Because of this limitation, the OD of the CT that can be run inhigh-pressure wells is limited. The smaller CT ODs have smallercross-sectional areas and thus require smaller snubbing forces whenfirst inserted into the well. However, the smaller CT can limit the workthat can be performed in a well. In many applications fluids are pumpedthrough the CT to clean sand or cuttings out of the well. The small CTID can restrict the flow rate of the pumped fluid.

[0043] A continuously varied outer diameter CT string according to thepresent invention can be used to improve this situation. The firstportion of the string has a small OD to meet the snubbing force limit.After some of the CT is hanging in the well, its hanging weight reducesthe snubbing force required and the OD of the string increases.

[0044] In one example a CT string is to be run into a 15,000 ft.vertical well with a wellhead pressure of 10,000 psi. Both the well andthe CT are filled with water with a density (ρ_(w)) of 8.34 lb/gal. TheCT injector is limited to a snubbing force of 15,000 lbs and is limitedin CT OD to 1.75″. The pump used for pumping the water through the CT islimited to a pump pressure of 15,000 psi. For simplicity, the stripperfriction or pack-off will be ignored. The CT material yield stress is100,000 psi. The combined Von Mises stress will be limited to 80% of theyield stress.

[0045] The maximum OD CT that can be snubbed against this wellheadpressure is 1.382″. FIG. 8 shows the profile for a VODCT stringaccording to the present invention that can be run into this well whilepumping at 15,000 psi. In the first 4,500 ft., the OD tapers from 1.382″to 1.75″. The diameter-to-thickness ratio is kept constant at 9.6. Inthe strings shown in FIGS. 7 and 8, the wall thickness variesproportionally to the outer diameter. It is within the scope of thisinvention to have a string or portion thereof with a wall thickness thatis constant or that varies porportionally to the outer diameter.

[0046] If a straight 15,000 ft. CT string with an OD of 1.382″ and awall thickness of 0.144″ is used on this well, the maximum flowrate ofwater that can be pumped through the string with 15,000 psi pumppressure is about 50 gal/min. If the VODCT string according to thepresent invention discussed above is used, the maximum flowrate is about90 gal/min. Thus, using the VODCT string according to the presentinvention increases the possible flowrate by 80%.

Production or Velocity String for Gas Wells

[0047] Often gas wells produce not only natural gas, but also some waterand/or liquid condensate. The tubing in a gas well is designed so thatthese produced liquids are carried to the surface and produced with theproduced gas. If the liquids are not produced they will collect or “loadup” in the well until the production of the well is impaired or stopped.To ensure that the liquids are produced, the tubing is designed so thatthe velocity of the gas is high enough to carry the produced liquids.Since the highest pressure in the well will be at the bottom, the tubingis selected to provide this high gas velocity at the bottom. As the gasmoves up the well, the pressure decreases and the volume of the gasincreases. If the tubing has a constant ID, the velocity of the gas mustincrease to handle the increased volume. By the time the gas approachesthe surface, the velocity may become very high, causing the upperportion of the tubing to choke the gas flow.

[0048] In methods and systems according to the present invention to dealwith these problems, a VODCT string according to the present inventionis used as the tubing string. Thus VODCT string is designed so thatinside area of the string increases as the pressure decreases, so thatthe velocity is constant or nearly constant. This VODCT string has alarger diameter at the surface that tapers to a smaller diameter at thebottom.

[0049] As gas wells age their bottom hole pressure decreases, causingthe gas velocity to decrease. If the gas velocity becomes too low, thewells load up with liquid. Often hanging a CT string in the well as abarrier to reduce the flow area rectifies this problem. Such a string iscalled a velocity string. The gas and liquids are usually produced upthe annulus between the velocity string and the tubing in the well. Toprevent the gas velocity from becoming too high near the surface, it maybe desirable for the annulus area to be smaller at the bottom and largerat the top. Prior art stepped CT velocity strings have been used forthis purpose, with the larger diameter section of CT at the bottom ofthe well and the smaller diameter section of the CT at the top of thewell. A VODCT string according to the present invention maintains a moreconstant velocity in the well. In one aspect such a VODCT stringaccording to the present invention has a smaller diameter at the surfacethat tapers to a larger diameter at the bottom.

Fracturing String

[0050] CT is used extensively in prior art methods for fracturingtreatments of wells. During a fracturing treatment a fluid carrying asand-like proppant is pumped through the CT and into the formation. Thevelocity of the proppant as it goes around bends in the CT often erodesthe CT material. Thus it is desirable to use large diameter CT wherethere are bends in the CT, e.g. but not limited to, at the surface. Thesize of CT that can be used is often limited by the size and weight ofthe reel on which the CT is transported. If the reel is too large it mayexceed road or crane weight limits, making it impractical or impossibleto use.

[0051] A VODCT string according to the present invention designed with alarger diameter at surface, tapering down to a smaller diameter in thewell (e.g. in part or in substantially all of a well) reduces thevelocity of the fracturing fluid and proppant as it passes through thebends in the CT (e.g. but not limited to bends at the surface), reducingor eliminating the undesirable erosion. The portion of the VODCT stringthat is in the well does not have any significant bends and thus canhandle a larger velocity without erosion. Most of the string (e.g. morethan 50%), in certain embodiments, may be of the smaller diameter,allowing the entire string to meet weight and size limitations.

Methods of Manufacture

[0052] In one method, according to the present invention, a strip forproducing CT according to the present invention (like the strip 103,FIG. 1) is cut with varying width so that when the strip is formed intoa tube it has the desired variable OD. This sub-process of cutting thestrip to a variable width is performed as part of the original stripslitting process, or as a separate process, or as an additionalsub-process in the strip assembly process (like that of FIG. 1), or in atube milling process (like that of FIG. 2) before a forming sub-process(like that of item 104, FIG. 2) or in the middle of the formingsub-process.

[0053] The varying strip width and corresponding varying OD requiresmodifications to several of the sub-processes in the prior art tubemilling process of FIG. 2. As shown in FIG. 9, in a method according tothe present invention, a strip 903 from a reel 902 enters a formingsub-process 904 capable of forming a tube 905 of varying OD. The firstfew rollers in the forming process are fixed forming rollers like thoseused for a constant OD CT string of the largest OD that will exist inthe VODCT string being built. These rollers are fixed in that theirshaping profile does not change throughout the length of the CT string.These fixed rollers shape the strip into approximately a U-shapedcross-section with a fixed curvature, no matter the width of the strip.Other items, apparatuses, and steps in the method of FIG. 9 correspondto similar items, etc. in the method of FIG. 2; e.g., the numerals 108and 908 idnicate the same seam normalizing step.

[0054] A next set of rollers are variable forming rollers as shown inFIG. 3. The strip 300 in FIG. 3 has almost completed the forming processand is nearly a tube. Several rollers 301 surround the strip/tube 300.The surface of these rollers may be either straight as shown, or concaveto fit the maximum OD of the string. These rollers 301 are held in yokes302 that can be moved radially to adjust for varying ODs. The yokes 302are connected to linear motion mechanisms such as pistons that move theyokes radially as required to vary the diameter of the string. Themechanism for moving the yokes is not shown. The rollers may be or maynot powered. The rollers may all be in one plane perpendicular to theaxis of the tube, or the rollers may be placed at various positionsaxially along the tube.

[0055] Variable dies, shown in FIG. 4A may be used in the formingprocess. The strip/tube 400 (like the strip/tube 300, FIG. 3) passesbetween two forming dies 401 and 402. The forming dies are rotated abouttheir centerlines to form the required diameter for that portion of thetube. A method according to the present invention may use the dies as inFIG. 4A or 4B and/or the rollers as in FIG. 3 to form the string.

[0056]FIG. 4B shows the strip/tube 400 passing between two forming dies410, 411. These dies are rotated as are the dies 401, 402 (FIG. 4A).

[0057] Either the variable OD forming rollers and the variable dies aredesigned so that the centerline of the tube stays in the same place forvarious tube diameters, or they are designed so that the edges of thestrip which will be welded remain at the same place for variousdiameters.

[0058] In the system of FIG. 4A, the centerline of the strip/tube ismaintained in the same position. In the system of FIG. 4B the point ofwelding (the top where the strip edges meet) is maintained at the sameposition. With the system of FIG. 3, the location of the centerline ofthe strip/tube depends on the roller location as adjusted and set by theinterconnected linear motion mechanisms of the yokes. In one aspect asthe string gets larger the top roller is maintained in a fixed positionand the bottom roller is moved down to maintain the weld location in thesame position.

[0059] A control system 917 or a plurality of control systems are usedto control the variable OD forming rollers and/or the variable OD dies.The control system(s) 917 measure how far apart the edges of the stripare, and adjust the rollers and/or dies to maintain the desired spacing.These control system(s) also ensure the correct amount of forging forceis applied to the edges of the CT in the welding sub-process 906.

[0060] In the welding sub-process 906 for the longitudinal seam the sizeof the impeder is varied by having a tapered impeder that is movedaxially during the welding process so that correct diameter ismaintained. The largest portion of the impeder must still fit throughthe smallest portion of the VODCT.

[0061] The sizing sub-process 910 works for the largest diameter of theVODCT string. In some cases, such as FIG. 8, a significant percentage ofthe string is at the maximum OD. With the rest of the VODCT string,maintaining the exact OD is not critical, so sizing is unnecessary.However, the sixing rollers also apply an axial force that helps movethe CT through the mill. In the case of VODCT, a pulling device in thesizing sub-process 910 such as powered rollers much like the variableforming rollers, or some other pulling device, may be added to apply therequired axial force. A completed tube 915 according to the presentinvention is spooled onto a reel 916.

[0062] In another method according to the present invention, a constantOD CT string is manufactured which is the maximum diameter of thedesired VODCT string. Then, either as part of the same tube millingprocess or as a separate process, the portions of the string that areintended to be a smaller diameter are drawn or rolled down to thedesired diameter. Many prior art patents disclose the general process ofdrawing flat stock, tubes, rods and the like through rotating dies,rolling mills, hot stretch millsand so forth. Typical of these prior artreferences are U.S. Pat. Nos. 3,783,663; 433,580; 860,879; 989,508;1,178,812 and 1,200,304.

[0063]FIG. 5 shows a drawing process according to the present inventionthat reduces the OD of a tube 500. A significant axial force is requiredto pull the tube 500 through the dies. Thus, the drawing process is madeup of a series of dies 501 which reduce the OD, and pullers 502 whichapply the required axial load. The number of dies and pullers neededdepend upon the amount the OD needs to be reduced. This process isperformed as a cold working process in which the tube is pulled throughthe dies cold, or it is a hot working process. In a hot working processthe tube is heated before being pulled through the dies.

[0064] Dies 501 may, according to the present invention, be variable ODdies as shown in FIG. 4A. This allows the same dies to be used for theentire operation. Alternatively, the dies 501 may, according to thepresent invention, be a series of fixed dies that are splitlongitudinally, so they may be removed when they are no longer needed.FIGS. 6A and 6B show such a series of die segments. The VODCT is beingreduced from a large diameter 600 to a smaller diameter 610 by a seriesof fixed dies 601 through 607. For the OD of the VODCT to increase, diesegments (607, then 606, etc.) are removed. This causes a fairly abruptbut small increase in diameter. Thus a VODCT string formed using aseries of dies like this according to the present invention has a seriesof small steps in the OD which form a nearly continuous OD taper.

[0065] Pullers may be powered rollers capable of variable diameters,like those shown in FIG. 3. Alternatively, CT injectors capable

[0066] The present invention, therefore, provides in certain, but notnecessarily all embodiments, coiled tubing with a first end spaced-apartfrom a second end, the coiled tubing having a first outer diamter at thefirst end and a second outer diameter at the second end, the first outerdiameter larger than the second outer diameter, and outer diameter ofthe coiled tubing continuously diminishing or increasng from the firstend to the second end or coiled tubing having a portion thereof withsuch a continuously varying outer diameter.

[0067] The present invention, therefore, provides in certain, but notnecessarily all embodiments, coiled tubing with a first end spaced-apartfrom a second end and an intermediate portion between the first end andthe second end, the coiled tubing having a first outer diameter at thefirst end, a second outer diameter at the second end, and a third outerdiameter at the intermediate portion, and outer diameter of the coiledtubing continuously varying from the first end to the intermediateportion and/or from the intermediate portion to the second end. Suchcoiled tubing may have: an outer diameter that continuously increases ordecreases from the intermediate portion to the second end and/or fromthe first end to the intermediate portion; and/or a wall thickness thatvaries, e.g., but not limited to, varying proportionally to the outerdiameter.

[0068] The present invention, therefore, provides in certain, but notnecessarily all embodiments, a method for pulling a coiled tubing stringfrom (or pushing it into) a wellbore (or form a pipeline) extending fromearth surface down into the earth, the method including pulling thecoiled tubing from the wellbore, the coiled tubing as any disclosedherein according to the present invention, e.g. but not limited to witha continuously diminishing outer diameter from the earth surface to itslower end; and/or wherein the coiled tubing has material with a yieldstress and an axial force is imposed on the coiled tubing, the axialforce causing an axial stress in the coiled tubing material, and whereinthe axial stress is limited to 80%, 60% or 40% of the yield stress ofthe coiled tubing material by sizing the outer diameter of the coiledtubing.

[0069] The present invention, therefore, provides in certain, but notnecessarily all embodiments, a method for snubbing coiled tubing into awellbore extending from earth surface down into the earth, the methodincluding introducing coiled tubing into the wellbore, the coiled tubinghaving an outer diameter of the coiled tubing continuously diminishingfrom a first end to a second end thereof and the method includingintroducing the second end first into the wellbore; and such a methodwherein an intermediate point on the coiled tubing is between the firstend and the second end and the weight of the coiled tubing between theintermediate point and the second end counters snubbing force requiredfor insertion of the coiled tubing into the wellbore. Simliarly thepresent invention provides, in certain if not all embodiments, a methodfor pulling or pushing a coiled tubing string into or out of a pipelineor into or out of a wellbore extending from the earth surface down intothe earth.

[0070] The present invention, therefore, provides in certain, but notnecessarily all embodiments, a method for producing a fluid from awellbore, the wellbore extending down from an earth surface into theearth, the fluid containing gas and liquid, the method includinginstalling coiled tubing in the wellbore, the coiled tubing comprisingthe coiled tubing having a first outer diamter at the first end and asecond outer diameter at the second end, the first outer diameter largerthan the second outer diameter, and outer diameter of the coiled tubingcontinuously diminishing from the first end to the second end, and thesecond end below the first end in the wellbore.

[0071] The present invention, therefore, provides in certain, but notnecessarily all embodiments, a method for producing a fluid from awellbore, the wellbore extending down from an earth surface into theearth, the fluid containing gas and liquid, the method includinginstalling coiled tubing in the wellbore, the coiled tubing comprisingthe coiled tubing having a first outer diamter at the first end and asecond outer diameter at the second end, the first outer diameter largerthan the second outer diameter, and outer diameter of the coiled tubingcontinuously increasing from the first end to the second end, and thesecond end below the first end in the wellbore.

[0072] The present invention, therefore, provides in certain, but notnecessarily all embodiments, a method for introducing fracturingtreatment material into an earth formation, the method including pumpinga fluid with fracturing treatment material down into a tubular stringextending down from an earth surface into an earth formation, the stringhaving a first portion with a first diameter at the earth surface and asecond portion with a second diameter at a portion of the string withinthe earth formation, the string having a continuously varying diameterfrom the first portion to the second portion, the first diameter smallerthan the second diameter, and the velocity of the fluid with fracturingtreatment material less when it flows in first portion of the stringthan in the second portion of the string; such a method wherein therelative lower velocity of fluid in the first portion is effective sothat erosion of the first portion of the string by the fluid withfracturing treatment material is less than that in the second portion ofthe string; and/or such a method wherein the second portion compirisesmore than 50% of the string; and such a method in which a realtivelylarger diameter string portion is provided for parts of the string thatare bent, e.g., but not limited to, portions of tubing at the surfacethat have bends therein.

[0073] In conclusion, therefore, it is seen that the present inventionand the embodiments disclosed herein and those covered by the appendedclaims are well adapted to carry out the objectives and obtain the endsset forth. Certain changes can be made in the subject matter withoutdeparting from the spirit and the scope of this invention. It isrealized that changes are possible within the scope of this inventionand it is further intended that each element or step recited in any ofthe following claims is to be understood as referring to all equivalentelements or steps. The following claims are intended to cover theinvention as broadly as legally possible in whatever form it may beutilized. The invention claimed herein is new and novel in accordancewith 35 U.S.C. §102 and satisfies the conditions for patentability in§102. The invention claimed herein is not obvious in accordance with 35U.S.C. §103 and satisfies the conditions for patentability in §103. Thisspecification and the claims that follow are in accordance with all ofthe requirements of 35 U.S.C. §112. The inventors may rely on theDoctrine of Equivalents to determine and assess the scope of theirinvention and of the claims that follow as they may pertain to apparatusnot materially departing from, but outside of, the literal scope of theinvention as set forth in the following claims.

What is claimed is:
 1. Coiled tubing comprising a first end spaced-apartfrom a second end, the coiled tubing having a first outer diamter at thefirst end and a second outer diameter at the second end, the first outerdiameter larger than the second outer diameter, and outer diameter ofthe coiled tubing continuously diminishing from the first end to thesecond end.
 2. The coiled tubing of claim 1 wherein wall thickness ofthe coiled tubing varies.
 3. The coiled tubing of claim 1 wherein wallthickness of the coiled tubing varies proportionally to the outerdiameter.
 4. The coiled tubing of claim 1 wherein wall thickness of thecoiled tubing is constant.
 5. Coiled tubing comprising a first endspaced-apart from a second end and an intermediate portion between thefirst end and the second end, the coiled tubing having a first outerdiameter at the first end, a second outer diameter at the second end,and a third outer diameter at the intermediate portion, and outerdiameter of the coiled tubing continuously varying from the first end tothe intermediate portion.
 6. The coiled tubing of claim 5 furthercomprising outer diameter of the coiled tubing continuously varying fromthe intermediate portion to the second end.
 7. The coiled tubing ofcliam 6 wherein the first outer diameter is greater than the third outerdiameter.
 8. The coiled tubing of cliam 6 wherein the third outerdiameter is greater than the second outer diameter.
 9. The coiled tubingof cliam 5 wherein outer diameter of the coiled tubing continuouslyvaries from the intermediate portion to the second end.
 10. The coiledtubing of claim 5 wherein wall thickness of the coiled tubingcontinuously varies.
 11. The coiled tubing of claim 5 wherein wallthickness of the coiled tubing continuously varies proportionally to theouter diameter.
 12. The coiled tubing of claim 5 wherein wall thicknessof the coiled tubing is constant.
 13. A tubular string, the tubularstring comprising a first portion with a first diameter a second portionwith a second diameter, the tubular string having a continuously varyingdiameter from the first portion to the second portion, the firstdiameter larger than the second diameter.
 14. The tubular string ofclaim 13 wherein the the tubular string is comprised of coiled tubing.15. The tubular string of claim 13 wherein the second portion comprisesmore than 50% of the string.
 16. Any patentable invention disclosedherein.